The invention disclosed herein is intended to be an improvement to a system of the type described in Applicant's Finnish publication No. 50,651, of which U.S. patent application Ser. No. 310,805, filed Nov. 30, 1972, now abandoned, is a counterpart. This system, which is known and will be referred to herein as the "Sym-Press" press section, includes a closed transfer press section which is a development which has been substantially responsible for recent increases in the operating speeds of paper machines. A reason for this is that, in addition to efficient dewatering, the Sym-Press section has the further advantage that web breaks are virtually eliminated.
However, in the practice of further raising paper machine speeds, the free runs of the web following after the press section have now become a bottleneck, the free runs or open draws referred to carrying the web either from the press section to the drying section or through the first free interstices in the drying section. Attempts have been made to avoid the web breaks at these points, and to avoid the consequent shutdowns of equipment by various techniques including by means of the expedients taught in Finnish Pat. No. 45,558 corresponding to U.S. Pat. No. 4,075,768; and Finnish patent applications No. 3720/74 and No. 761114, both corresponding to U.S. Pat. No. 4,016,032. While these techniques have resulted in improvements, they have not completely eliminated web breaks occurring after the press section, which breaks have harmful effects on run ability of the paper machine.
Accordingly, a primary object of the present invention is to provide a method and apparatus by which the running speed and reliability of a paper machine can be improved by enhancing the dewatering action in the press section so that the web leaves the press section having a higher dryness and higher strength than in previous devices.
It is appropriate to observe in this connection that the object of the invention is not so much to achieve in paper machines the optimum condition as regards heat economy, but rather to optimize the operation economy of the paper machine in view of its running efficiency and reliability in operation.
It is a well-known physical fact that the viscosity of water considerably decreases with increasing temperature of the water. This phenomenon has been utilized in paper machines in order to enhance the dewatering of the paper web. As examples of these prior art approaches, reference is made to previously issued U.S. Pat. Nos. 2,907,690; 3,097,994; 3,560,333; and 3,655,507. Of these, the first mentioned U.S. Pat. No. 2,907,690 discloses a normal paper machine press wherein a hot gas jet is directed from nozzles with small holes against the felt on the suction roll prior to the press nip, the intention being to lower the viscosity of the water residing in the felt and thereby to enhance the dewatering action.
The U.S. Pat. No. 3,097,994 also mentioned above discloses the use of steam supplying means in various parts of the paper machine and the means in question consists of a steam feeder box and a suction box opposed thereto. As regards the use of this particular design in the aid of enhanced dewatering in a press nip, the drawback to be observed is that the steam-supplying means has been placed rather a long distance before the conventional suction press nip, with the result that the web cools before it reaches the nip.
In the U.S. Pat. No. 3,560,333, there is disclosed means for the supplying of steam into the throat between the well-known Yankee cylinder and the suction roll opposing it, the aim being to prevent the cooling of Yankee cylinder at the press nips.
In the U.S. Pat. No. 3,655,507, a press nip with two felts is shown wherein pressurized steam-supplying means is provided inside one of the two rolls, the steam heating the felt for enhanced dewatering action in the subsequent nip.
The means for enhancement of dewatering which are based on the raising of the temperature of felt, web and/or roll have not gained very extensive application, particularly not in fast paper machines. This is partially due to the fact that it has not been possible, using the means of the prior art, to conduct to the points of supply a sufficiently high heat flow. This, again, is caused by the fact that, for example, to avoid destruction of the felt, the temperature of the treatment gas is limited and also because the available time is short because of the high web and felt speed, and further on account of space consideration, the heat transfer surface is also quite restricted. An aim of the present invention is to provide a solution to the problems presented.
Briefly described, the invention includes a process of enhancing the dewatering of a paper web in the press section of a paper machine of the type having at least two press nips through which the web consecutively passes, the first one of the nips being formed between a water-receiving roll and a suction roll and a subsequent one of the nips, in the direction of web travel, being defined by the suction roll and a plain roll, and wherein the web runs on the suction roll between the nips, the improvement comprising the step of treating the web by exposing the outer surface thereof to hot steam between the two nips so that a significant proportion of the treatment steam condenses thereon, thereby imparting the latent heat of the steam to the web.
The invention also includes apparatus for accomplishing this method in a machine of the type described and includes steam supply means mounted adjacent the web and means for conducting steam to the supply means, the supply means comprising at least one steam supply box extending across the entire width of the web adjacent the suction roll, the box having an interior space and an open side opening toward the suction roll and conforming to the curvature of the roll over a substantial sector thereof to define a flow path for treatment steam onto the sector.
Since according to the invention hot steam is used to raise the temperature of the web, the steam will in its condensation process release and impart to the web its considerable latent heat, in the order of 2,270 kilojoules per kilogram (kJ/kg). When, furthermore, the point where the steam is supplied is located as taught by the invention, the supply surface can be made rather large and the steam can be induced to penetrate immediately into the web which is being treated and also partly into the felt lying thereunder quite efficiently, also utilizing the vacuum zone at this part of the suction roll of the press. It is a further advantage of the invention that, exactly at the point of supply specified in the invention, adequate space exists for expedient steam supply means. It is also an advantage that when according to the invention the point of steam supply is placed after the first nip, then the web has had time when entering that nip, which is advantageously formed between two felts and in which the web is efficiently dewatered in two directions, to attain a dry matter content (in the order of 20 to 35%) such that one need not heat unnecessarily by the steam supply such water which is even otherwise easy to remove from the web.